In many circuits, it is necessary to generate a voltage which is greater in magnitude than the supply voltage. For example, in order to efficiently drive DMOS output structures of MOS based power IC's, it is necessary to supply a gate-to-source voltage of typically 10-15 volts. In many high-side drive and low-side drive intelligent power switches, a voltage tripler is normally required, as the bulk of intelligent power switch designs are for the automotive market, where operating voltage may be as low as 4.5 volts. At this voltage, most DMOS structures are not functional, or have a very high on resistance. A voltage doubler would normally provide about 7 volts, with a supply voltage of 4.5 volts, when diode and saturation losses are taken into account. This is normally insufficient to fully turn on a DMOS output, so a tripler is required. Other circuits, such as EPROMS and EEPROMS, require a programming voltage (V.sub.pp) of 12-17 volts to be generated from a five volt supply.
Typically, charge pumps (also referred to as "voltage multipliers") are used to generate a voltage of increased magnitude. Present day charge pumps comprise a series of stages, each stage including a capacitor and a MOS or junction diode. Each stage of the charge pump boosts the magnitude of the voltage signal by a voltage equal approximately to the voltage swing of a clock signal applied to the capacitor less the threshold voltage of the diode. At each stage, the capacitor will see an increased maximum voltage difference across its plates. For example, using a seven volt supply, the first capacitor will see a difference of seven volts across its plates and the capacitor of the second stage will see a maximum voltage difference of fourteen volts across its plates. Subsequent stages would result in additional increased voltage across the capacitors. The magnitude of voltage across the capacitor will determine its structure and size. Consequently, by limiting the voltage across the capacitors, the density of the circuit can be increased and the complexity of the fabrication may be decreased.
Therefore, a need has arisen in the industry for a charge pump using low voltage capacitors.